Some conventional motorcycles include systems in which a grip position sensor detects an opening degree of a throttle grip gripped by a driver and an ECU (electronic control unit) electronically controls a motor, which in turn causes a throttle valve to be opened and closed, based on a detection value of the grip position sensor. In these systems, since an optimal target opening degree of the throttle valve is calculated and the opening degree of the throttle valve is electronically controlled so that a deviation between an actual opening degree and a target opening degree is minimized, an amount of intake-air supplied to the engine is maintained at an optimal level.
In some of the above described systems, when a failure occurs in its control system, the target opening degree is instantly set to an idling opening degree corresponding to an idling engine speed irrespective of an amount of the driver's grip operation, and the throttle valve is forcibly closed at a highest rotational speed of the motor. In the case of a four-wheeled vehicle whose vehicle body has a large weight, even if the throttle valve is quickly closed, the resulting deceleration shock is not great because of a larger inertia force. On the other hand, in the case of a lightweight vehicle whose vehicle body has a small weight, if the throttle valve is quickly closed, the resulting deceleration shock is great because of a smaller inertia force. In this case, the driver may sometime feel driving discomfort, depending on a driving state at the detection of a failure. Accordingly, to avoid the driver feeling driving discomfort, there has been disclosed a system in which a vehicle speed is gradually decreased by controlling the speed at which the throttle valve is closed, at the detection of a failure.
However, in the above described conventional system, the speed at which the throttle valve is closed is constant at the detection of a failure, irrespective of the driving state of the motorcycle. This may sometimes make the driver feel driving discomfort depending on the driving state at the detection of a failure. For example, when the motorcycle is being accelerated at the detection of a failure, the driver may feel a relatively large deceleration shock. On the other hand, when the motorcycle is being decelerated at the detection of a failure, the driver may feel a relatively small deceleration shock, because the time taken for completing the deceleration and reaching an idling state tends to be long. Such a situation occurs in vehicles other than motorcycles.